In the leading-edge industries of semiconductor manufacturing/liquid crystal manufacturing, and the like, it is important to control the contamination of the air or the product surface in a clean room in order to ensure the yield, quality, and reliability of a product. In particular, in the semiconductor industry field, control of an ionic gaseous contaminant becomes indispensable in addition to the control of a particulate contaminant using a HEPA (High Efficiency Particulate Air) filter, a ULPA (Ultra Low Penetration Air) filter or the like, with an increase in integration degree of products. For example, amines are contained in an amount of several tens parts per billion in an atmosphere, and enter the clean room upon intake of an outside air. Further, it is known that basic gaseous contaminants such as amines are also generated from the construction materials of a clean room or the chemicals for processing.
When these basic gaseous contaminants are present in an amount of 10 to 20 ppb in a clean room, the defective shape of a photoresistance is caused (T-top phenomenon). Further, they react with an acidic gas, thereby to form a salt, so that fogging is caused on the surface of an optical component of a processing device or a silicon wafer. For this reason, an adsorption filter carrying an acidic substance as a reactive material for absorbing a gas is used to remove basic gaseous contaminants such as amines.
On the other hand, when acidic gaseous contaminants are present in a clean room, they corrode the glass fiber of a dust collecting filter, thereby to promote the generation of boron, and to bring about a metal corrosion of IC or the like. In order to take measures against such acidic gaseous contaminants, an adsorption filter carrying a basic substance as a reactive material for absorbing a gas is used to remove the acidic gaseous contaminants.
The adsorption filter may be used in such a manner as to be set in a clean booth or a ceiling-mounted FFU (Filter Fan Unit), or in such a manner as to be set in a semiconductor-manufacturing apparatus. In such a case, the reduction in thickness of the adsorption filter has proceeded with downsizing of an air conditioning unit.
Alternatively, the adsorption filter may be set in a return duct in a clean room, and at an outside air inlet. In this case, the adsorption filter is required to be housed in a predetermined space. On the other hand, at the return duct in the clean room, and at the outside air inlet, it is required to treat a large quantity of air containing impurities. This inevitably results in a high wind velocity in the duct. Accordingly, a small thickness filter provides a short contact time (residence time) with the air to be treated, so that a sufficient removing performance cannot be obtained. When the filter thickness is increased in order to increase the residence time, the pressure loss increases. This unfavorably causes problems of necessitating an increased capacity of an air blower, an increased blowing energy, and the like. Accordingly, there are demands for the reduction in area occupied by the filter, and the reduction in pressure loss of the air conditioning equipment system.
Therefore, as the method for satisfying the requirements for the adsorption filter for use in the return duct in the clean room, and at the outside air inlet, there is a method in which the filter surface area is increased relative to the duct opening area, thereby to increase the residence time. Specifically, there are a method in which several sheets of filters are housed in a casing, and disposed to make a plurality of V configurations (see Patent Document 1), and a method using a pleated type filter (see Patent documents 2 and 3), which are adaptable to high wind velocity (wind velocity 1 m/s to 3 m/s).
Patent Document 1: JP 9-220426 A
Patent Document 2: JP 9-239223 A
Patent Document 3: JP 7-289828 A
Incidentally, when filters are of a cassette type filled with particulate activated carbon, and disposed in a V shape, according to a conventional technique, a plurality of small thickness filters are manufactured, and the filters are respectively framed, and then housed in a casing, and disposed to make a plurality of V configurations. In this case, there are the following problems: the powder falling amount and the generated dust amount are large, resulting in a contamination in a clean room; the weight is very large because an activated carbon is filled therein; framing of the filter is time-consuming; the total weight has been increased by the weight of the cassette frame; and other problems.
On the other hand, in the pleated structure, the filter medium is a felt-like or nonwoven fabric-like one. A corrugated spacer for shape keeping, or a hard porous material such as a web made of a metal or made of a hard plastic or a punching metal is generally used therefor, which causes an increase in pressure loss. Further, there is also another problem as follows: an air does not flow through the crest and valley portions of the folds, so that the filter is not consumed uniformly (see Patent Document 3). Still further, the joint portion between the spacer for shape keeping and the filter medium is required to be welded by an adhesive. Furthermore, assembling is not easy, so that the corrugated spacer and the hard porous material become industrial wastes upon disposal (see Patent Document 2).